Whilst many modern high performance applications utilizing digital systems require accurate reproduction of material that has a wide frequency range, typically 20 Hz-20 kHz, the range of 150 Hz-5 kH can be considered most important as it contains the range of maximum acuity of human ear, and the greatest concentration of information in normal program material.
Although, it is well recognized that a single transducer represents the ideal arrangement in providing a coherent point source wave front, none is available for accurately and efficiently reproducing the above range of 150 Hz-5 kHz let alone 20 Hz-20 kHz, at high sound pressure levels. Inherent limitations, notably, distortion of the higher frequencies by movement of the transducer cone through the extension required to reproduce the lower frequencies, restrict the lower range of the mid range speaker to a lower limit of 250 Hz, while reflections off the magnet, resonances around the apex of the cone and under the conventional dust cap, restrict the upper limit to 1-2 kHz. Consequently, technicians have generally accepted the restriction of a single mid range transducer to 250 Hz-2 kHz. Two further transducers, differently sized from the first mentioned transducer, are then required in such systems to cover the remaining frequency ranges, usually a 15-18 inch woofer and a 2-4 inch diaphragm high compression driver mated to a horn with two inch throat for high frequencies. However, such arrangement necessitates two crossovers, at 250 Hz and 2 kHz, respectively, between transducers of different sizes and therefore different transient responses, between 150 Hz and 350 Hz at the low end and between 1 kHz and 3 kHz at the high end, the latter also experiencing other distortions inherent at lower operating ranges of high frequency compression drivers and with a roll-off at, typically, 16 kHz.
It is well established that the different transient responses are a consequence of the speaker diaphragms of differing size and mass, the larger diaphragm being transiently slower so that it is still resonating from an earlier impulse when a subsequent impulse containing the same or another frequency is generated, so that, adjacent the crossover, where the same frequencies are generated, the larger transducer resonates over the transiently quicker response of the smaller transducer providing audible blurring. The result is a systemic "flat" frequency response consisting of parallel resonances in time created by electrical impulses arriving at different times from different transducers.
In one attempt to provide a single point source covering the vocal range with crossover points at 150 Hz and 600 Hz, U.S. Pat. No. 5,004,067 issued Apr. 2, 1991 to Patronis, teaches a horn loaded system for horizontally controlled cinema application in which a high frequency driver is mounted coaxially in front of a mid frequency loudspeaker wherein a portion of the high frequency driver acts as a phase plug for the driver of the mid frequency speaker to achieve controlled dispersion and increase efficiency. However, the patent teaching directs that the high frequency driver operate down to a frequency even lower than that of the conventional system, which would normally introduce a risk of commensurate significant distortion within the high frequency horn, and, actually, requires electronic equalization to compensate for roll-off. Furthermore, the diaphragm size difference of the drivers at the crossover taught is too great to avoid blurring. However, in a development of the teaching of U.S. Pat. No. 5,004,067, sold as the Pro-Ax series model PX-1060, by OAP Audio of Buford, Georgia, which is intended for use in a high performance environment, Patronis requires that the crossover point be raised from 600 Hz to 1250 Hz utilizing only a single 10 inch horn loaded transducer to reproduce the range of 250 Hz to 1250 Hz on the understanding that a separately housed subsystem would also be utilized for lower frequencies, which, of course reverts to the traditional two crossover points within the 150 Hz to 5 kHz range,-with their attendant disadvantages described above. In other respects the speakers system taught by Patronis follows the common approach of providing a horn mouth wider than it is tall to support a more controlled horizontal response, whereas in an auditorium for sound reproduction a tighter control of the vertical response is believed desirable to reduce reflections from the ceiling which is usually the nearest surface.
U.S. Pat. No. 4,836,327 issued 1989 to Andrews et al also teaches a coaxial assembly of mid and high frequency speakers but the high frequency driver is located behind the low frequency driver in substantially non-loading relation.
In another attempt to produce a single point source using multiple transducers taught by U.S. Pat. No. 5,526,456 issued Jun. 11, 1996 to Heinz, the high frequencies are modulated by the low frequencies produced in a common throat leading to distortion while the horn is of relatively complex form requiring precise expansion ratios and, therefore, expensive in design, if degradation is to be avoided. U.S. Pat. No. 4,391,346 issued 1983 to Murakami and U.S. Pat. No. 4,733,749 issued 1988 to Newman et al also teach a bass-reflex speaker for emulating a single point source by using multiple transducers, but only for frequencies in the low range.